In general, liquid developers for electrophotographic purposes are obtained by dispersion organic or inorganic pigments or dyes, such as carbon black, nigrosine or phthalocyanine blue, for example, and natural or synthetic resins, such as alkyd resins, acrylic resins, rosin or synthetic rubbers, for example, in a liquid which has good electrically insulating properties and a low dielectric constant, such as a petroleum based aliphatic hydrocarbon, and adding polarity controlling agents such as metal soaps, lecithin, linseed oil, higher fatty acids or polymers which contain vinylpyrrolidone, for example.
In developers of this type, the resin is dispersed in the form of insoluble latex particles with a particle diameter from a few nm to a few hundred nm. However, in a conventional liquid developer the bonding between the soluble resin, which is used for dispersion stabilization purposes, or the polarity controlling agent and the insoluble latex particles is insufficient, as a result, the soluble resin for dispersion stabilization purposes or the polarity controlling agent readily diffuses into the solvent. Consequently, the soluble resin for dispersion stabilization purposes becomes separated from the insoluble latex particles. On long term storage or repeated use, the particles may sediment, coagulate or lump together, and the polarity becomes indistinct. Furthermore, it is difficult to redisperse particles once they have been sedimented or formed into lumps, as a result they tend to become attached to certain parts of the developing apparatus and they may contaminate the image parts or cause a breakdown of the developing machine by blocking pumps, for example.
It has been suggested that the insoluble latex particles should be chemically bound to the soluble resin for dispersion stabilization purposes in an attempt to eliminate these disadvantages, and disclosures to this effect have been made, for example, in U.S. Pat. No. 3,990,980. However, although such liquid developers are somewhat better in terms of their dispersion stability with respect to the natural sedimentation of the particles, this effect is not sufficient. The redispersion stability still is insufficient. Moreover, when these developers are used in actual developing apparatus there is a problem in that the toner which becomes attached to various parts of the apparatus solidifies in a film like form from which redispersion is difficult. This can lead to a breakdown of the apparatus and contamination of the transferred images for example. Furthermore, the combinations of dispersion stabilizers and insolubilized monomers which can be used to prepare mono-disperse particles with a narrow particle size distribution is very limited in the methods of manufacture of resin particles disclosed in the above mentioned documents. They tend to be polydisperse particles which have a wide particle size distribution including large numbers of large, coarse particles or in which two or more average particle sizes are present. Furthermore, it is difficult to obtain particles of the prescribed average particle size in a mono-dispersion which has a narrow particle size distribution, and large particles of at least 1 .mu.m, or very fine particles of less than 0.1 .mu.m, are formed. Moreover, there is a further problem in that the dispersion stabilizers which are used must be prepared using a complicated and time consuming process.
Additionally, methods of overcoming the above mentioned problems in which the degree of dispersion of the particles, the redispersion properties and the storage properties are improved by using insoluble dispersed resin particles of copolymers of insolubilized monomers and monomers which contain long chain alkyl groups or monomers which contain two or more polar components were disclosed, for example, in JP-A-No. 60-179751 and JP-A-No. 62-151868. (The term "JP-A" as used herein signifies an "unexamined published Japanese patent application".)
On the other hand, techniques in which more than 5000 copies are printed using offset printing master plates obtained using electrophotographic techniques have been introduced in recent years. In particular, progress has been made in improving the master plates so that it is now possible to print in excess of 10,000 copies even with large plate sizes. Furthermore, progress has been made in shortening the operating time of the electrophotographic plate making system where improvements have been achieved in speeding up the development/fixing processes.
The dispersed resin particles manufactured using the procedures disclosed in the aforementioned JP-A-No. 60-179751 and JP-A-No. 62-151868 do not always provide satisfactory performance in terms of particle dispersion properties and redispersion properties when development speeds are increased and in terms of printing resistance when the fixing time is shortened or when the master plate is large (for example A3 size or greater).
The problems encountered with conventional liquid developers of the type described above are solved by the present invention.